1. Field of the Invention
The present invention relates to an improved system and method for controlling vehicle speed so as to trail a preceding vehicle while maintaining a safe intervehicle distance between the vehicle and preceding vehicle.
2. Description of the Prior Art
As an adjunct to recently popular "cruise control" systems which automatically hold the vehicle at a constant set speed, various automatic systems have been proposed for trailing preceding vehicles travelling in front of the subject vehicle and in the same direction on the basis of detected intervehicle distance and relative speed. One of the above-mentioned automatic trailing systems is exemplified by Japanese Utility Model Application Unexamined Open No. Sho. 48-99,085 published on Nov. 22, 1973. In the disclosed automatic trailing system, additional means for detecting vehicle speed relative to the preceding vehicle is separate from means for detecting the vehicle speed and means for detecting intervehicle distance. Therefore, the structure of the system is complex and the reliability of the system is reduced. In addition, the increase in the number of system components causes an increase in cost.
To correct for these disadvantages, a conventional system has been proposed in which the relative speed is calculated from the rate of change of intervehicle distance (.DELTA.l) detected by the intervehicle distance detecting means over a unit time (.DELTA.t) so that the above-described means for directly detecting the relative speed is not required.
However, another problem arises in the latter system. Specifically, in the conventional method of calculating the relative speed from the rate of change (.DELTA.l) in the intervehicle distance per unit of time (.DELTA.t), the responsiveness of the system during measurement of relative speed is reduced if a long unit of time is selected and, on the other hand, the accuracy of the measured value of the relative speed is reduced if a short unit of time (.DELTA.t) is selected. Therefore, it is impossible to achieve a system which has both the responsiveness of the conventional system in which a additional relative speed detecting means is used and the high accuracy of the relative speed, although the system is simpler in structure and less expensive.